Rainbow of rocks: mysteries of sandstone colors and concretions in Colorado Plateau canyon country

Sunrise illuminates Colorado Plateau’s canyon country. In the early morning light, cliffs radiate a rich red glow, and a sculptured panorama of sandstone is revealed in a rich palette of crimson, vermilion, orange, salmon, peach, pink, gold, yellow, and white. Nearby are black, spherical rock marbles (iron concretions) collecting in small depressions, like puddles of ball bearings. These natural spherical balls have been called various names such as iron nodules, iron sandstone balls, or moki marbles. However, we use the name “iron concretion” to describe both the composition (iron oxide that is the dark mineral which cements the sandstone grains) and the formed shape (concretion).

What paints the sandstone such rich colors? Why is red a dominant color? Where do the black marbles come from? How did the black marbles form? Is there a relationship between sandstone colors and the marbles? This booklet explores the answers to these questions and poses other questions yet unanswered.